In women undergoing ovarian stimulation for IVF, we hypothesize that 1) inhibin B is a marker of the size of the cohort of intermediate-size (10-15 mm) follicles, 2) inhibin A is a marker of the size of the cohort of large (greater than 15 mm) follicles, 3) inhibin A is a marker of the number of mature oocytes retrieved, and 4) inhibin A is a better marker of oocyte maturity than serum E2. Our specific aims to test these hypotheses are 1) charaterize the pattern of inhibin B and inhibin A secretion during ovarian stimulation for IVF, and to determine the degree to which serum inhibin B, inhibin A, and E2 levels on the day of hCG administration correlate with the numbers of intermediate-size and large follicles, mature oocytes retrieved and fertilized as well as embryos transferred. Twenty-two women signed consent to participate in this project; 2 women were deemed screen failures, 4 women withdrew consent. We have been approved for 48 women to participate in this trial. We anticipate that to identify and recruit an additional 32 women will take at least an additional year. The future implications of this study are the development of clinical strategies for the management of ovarian stimulation for IVF that utilize inhibins as markers of follicle and oocyte maturity to maximize the numbers of mature oocyte retrieved, minimize the risks of OHSS and IVF cycle cancellation, and predict the need for in vitro oocyte maturation.